The propeller assembly of a marine vessel typically rotates at variable speed, both in the forward and reverse directions. Where a marine vessel uses an electric power and propulsion system then the motor driving the propeller assembly will also need to operate at variable frequency (for the case where the propeller assembly is driven directly by the rotor of the motor then the frequency will be approximately proportional to the speed of rotation of the propeller assembly) and variable voltage (approximately proportional to frequency). The power system of the marine vessel will have a nominally fixed voltage and frequency and it is therefore necessary to interface the motor to the power system using a power converter.
The power converter will typically consist of two parts: a network bridge that rectifies the ac power from the power system to a nominally fixed dc voltage (the dc link), and a motor bridge that inverts the dc voltage to the appropriate ac voltage for the motor. The power systems of many marine vessels are often referred to as being “weak” since the total generating capacity is closely matched to the total load. This means that when large loads connected to the power and propulsion system are turned on, significant transients (dips) may occur. Any dips or faults on the power system will interfere with the network bridge and its operation to provide the dc voltage. It is therefore normal for the power converter to be turned off to avoid damaging the various components. For many marine applications this requirement to turn off the power converter, and hence the total loss of the associated propulsion equipment, is considered unacceptable.
There is therefore a need for an improved power converter that can remain connected to the power system in the event of a dip or a fault.